Stimulation treatments may be performed in order to enhance production from a fluid filled borehole such as an oil or gas well. Examples of stimulation treatments include hydraulic fracturing (fracing) and insertion of materials and chemical substances which alter either or both of the formation and the perforations in the well completion, e.g., an acid ballout. The intended result of the treatment can be to enhance production by altering the perforations, altering the formation adjacent to the perforations, creating new perforations, or combinations thereof.
Chemical or mechanical sealants are often used to alter hydraulic connectivity at various locations in the borehole system during stimulation treatments. For example, at each stage in a multiple stage fracture treatment the previously fractured zones are isolated by pumping diversion fluid or ball sealers into the borehole from the surface in order to seal the perforations of the previously fractured zone. In a simple acid ballout treatment the sealants are used to seal those perforations which already have low hydraulic resistance to flow. Acid is then pumped into the zone to reduce hydraulic resistance at non-sealed perforations. Alternatively, new perforations are created by pumping fluid into the zone in order to increase pressure to a point of opening perforations, slots or higher pressure intervals.
The state of the sealants and perforations during stimulation affects the outcome of the treatment. In particular, because unsealed perforations are in hydraulic communication with one another, treatment of target perforations can degrade or fail if sealing of other perforations fails during pumping. One problem faced by operators is that determining the state of sealants and perforations can be difficult or impossible. In the case of an acid treatment, it is difficult or impossible to determine whether the fluid and pressure are cleaning the intended zone or flowing past a failed seal. Similarly, in the case of fracing, it is difficult or impossible to determine whether particular zones or perforations are opening a new fracture or expanding an old fracture behind a failed seal.
With regard to determining conditions in a fluid-filled borehole, U.S. patent application Ser. No. 11/691,071 entitled WIRELESS LOGGING OF FLUID FILLED BOREHOLES, filed 31 Oct. 2007, incorporated by reference, describes locating and monitoring changes in downhole conditions by recording, generating and analyzing tubewaves propagating in the borehole system.